1. Field of the Invention
This invention relates to encapsulating compositions for electronic devices such as semiconductor devices, integrated circuits and microelectronic components.
2. Description of the Related Art
Resin compositions, particularly based on silicone resins or epoxy resins, are well known for encapsulating electronic devices. It is also well known that such compositions are often contaminated by trace amounts of metal ions, particularly alkali metal ions. If such metal ions are permitted to migrate in the composition, they can adversely affect the performance of the electronic device and may ultimately lead to device failure. Chloride ions can also give rise to a corrosion problem.
It has been suggested to incorporate a crown ether or cryptate ether in such compositions as metal ion scavengers (U.S. Pat. Nos. 4271425 and 4278784 of Ching-Ping Wong, referring to Japanese Patent No. 76-11377 of Kaneda et. al.). However crown ethers and cryptate ethers are unsuitable for commercial use because of their high toxicity.
U.S. Pat. No. 4556700 of S. Harris et. al. describes the use in adhesive compositions of calixarene compounds represented by the formula ##STR2## where R.sup.1 is alkyl, alkoxy, substituted alkyl or substituted alkoxy; R.sup.2 is H or alkyl; and n'=4, 6 or 8. It had previously been suggested to use crown ethers as accelerators in these compositions.
McKervey et. al., J. Chem. Soc. Commun. 1985 p. 388 describes the cation transfer properties of alkyl calixaryl acetates. These ester calixarenes are known to be capable of sequestering alkali metals. However the behavior of calixarene derivatives in the environment of an encapsulating composition could not be predicted. The present inventors have found that certain calixarene derivatives can scavenge alkali metal ions so efficiently that an epoxy resin which would not normally be regarded as useful for electronic applications can be converted to a material having a relatively low level of contamination.
The same paper by McKervey et al compares the selectivity towards alkali metal picrates of alkyl calixaryl acetates, 18-crown-6, and a silacrown. The results published in that paper suggest that the silacrown is substantially less efficient for ion transport and has low sodium sequestering ability, as compared to the calixarene derivatives and 18-crown-6. We have now unexpectedly found that silacrowns have a level of activity comparable to 18-crown-6 and calixarene derivatives as alkali metal ion scavengers in encapsulating compositions. This illustrates the unpredictable nature of metal scavenging ability in encapsulating compositions. We have identified a range of compounds which are suitable as metal ion scavengers in such compositions.